The metallic materials of industrial cooling and/or heat-transfer systems, especially in internal combustion engines, are subject to chemical and mechanical stresses when aqueous, glycol-containing heat-transfer or cooling media are used. These produce more or less grave damage to the material, i.e. corrosion and cavitation. The degree of damage depends on the quality of the metals, the cooling fluid, the heat-transfer fluid, and on the external conditions of the cooling cycle. The damage to the material can be effectively combated and reduced by adding suitable inhibitors to the functional fluids, but can be entirely prevented only in the rarest cases. Preferred corrosion-protecting agents are benzoates, borax, nitrites and, occasionally, also phosphates. The effect of the inhibitors is specific and normally is restricted to one metal group when used within a limited concentration range. As the metals are exposed to increased stress, for example due to cavitation, the inhibitors may in certain cases lose their beneficial protective effect and increased damage to the materials will ensue. In these cases, the damage to certain metals, for example aluminum, can be higher than that which would have occurred in the absence of any inhibitor ingredient.
Due to conditions of construction, cavitation damage is primarily observed on components made of cast iron and aluminum, e.g., on pumps. Such damage occurs predominantly in cooling systems for internal combustion engines. In addition to the water pump, also endangered are the cylinder liners, parts of the crankcase, radiators, and other components. Accordingly, principally, cast iron and aluminum and its alloys are to be protected against cavitation. The conventional corrosion-protecting additives are suitable for this purpose only in exceptional cases.
Borate-nitrite mixtures have a cavitation-inhibiting effect in water with respect to gray cast iron. However, in glycol-water mixtures, this inhibitor combination results in increased erosion of aluminum. Inhibition with sodium benzoate and sodium nitrite or sodium nitrate likewise still permits unduly high cavitation erosions on aluminum and its alloys. Moreover, these formulations are sensitive to certain chemicals, such as, for example, sodium carbonate, which are frequently utilized to set a minimum alkalinity of the coolants and for protection against superacidification.
Cavitation-inhibiting cooling liquids for diesel engines are known from DAS's [German Published Applications] Nos. 1,239,137 and 1,263,398. They contain polyalkylene glycols or polyoxethylates of alcohols, carboxylic acids or carboxylic acid amides. Cavitation-inhibiting cooling fluids for internal combustion engines are also known from DAS No. 1,239,138. These contain polyvinylpyrrolidone.
However, the prior art additives are effective only on gray cast iron and, even in this case, only when used in comparatively high concentrations (0.1-5% by weight). These additives are practically ineffective on aluminum, as will be demonstrated below by comparative tests (Table 1). This is especially disadvantageous since it remains a prime objective to improve cavitation protection of aluminum and aluminum alloys in view of the increased usage of these light metals in engine construction and in heat-transfer e.g., (solar) installations.